High vacuum gettering pump

ABSTRACT

A HIGH VACUUM GETTERING PUMP IS DESCRIBED WHICH IS CAPABLE OF PROVIDING A HIGH VAPORIZATION RATE OF A GETTER MATERIAL. THE PUMP INCLUDES A HIGH THERMIONIC EMISSION CATHODE FILAMENT DISPOSED OVER A SURFACE OF A BULK SUPPLY OF A GETTER MATERAL SUCH AS TITANIUM. THE GETTER MATERIAL IS CONTAINED WITHIN A CUP-SHAPED HEAT SHIELD AND HAS A PERIPHERAL CONFIGURATION MINIMIZING THE AMOUNT OF MATERIAL AT THE SIDES OF THE BODY OF GETTER MATERIAL WHICH WILL NOT BE MELTED BY THERMIONIC EMISSION FROM THE FILAMENT WHEN A HIGH OPERATING POTENTIAL IS APPLIED BETWEEN THE FILAMENT AND THE GETTER MATERIAL BODY. A SHIELD IS ALSO   PROVIDED BLOCKING LINE-OF-SIGHT TRAVEL OF VAPORIZED GETTER MATERIAL TO A PORTION OF THE FILAMENT WHICH DOES NOT REACH A HIGH ENOUGH TEMPERATURE TO PREVENT CONDENSATION OF THE GETTER MATERIAL. THE SHEILD IS TAPERED INWARDLY AND CURVED AWAY FROM THE FILAMENT ADJACENT AN UNSHIELDED PORTION THEREOF TO MINIMIZE THE EFFECT CAUSED ON THE OPERATION OF THE FILAMENT BY THE BUILD-UP OF GETTER MATERIAL ON THE SHIELD.

0, 1972 o. D. ROBERTSON ET AL 3,666,377

HIGH VACUUM GETTERING PUMP Filed June 2. 1969 2 Sheets-Sheet 1 FIG! INVENTORS DAVID D ROBERTSON WILLIAM R. WHEELER BY q urfl l: W ATTORNEY May 30, 1972 Filed June 2. 1969 D. o. ROBERTSON ETAL 3,666,377

HIGH VACUUM GETTERING PUMP 2 Sheets-Sheet INVENTORS DAVID D. ROBERTSON WILLIAM R WHEELER BY 1' ATTORNEY United Smtes Patent ie 3,666,377 HIGH VACUUM GEITERING PUMP David D. Robertson, Palo Alto, and William R. Wheeler,

Saratoga, Calif., assignors to Varian Associates, Palo Alto, Calif.

Filed June 2, 1969, Ser. No. 829,386 Int. Cl. F04!) 37/02 US. Cl. 417-49 15 Claims ABSTRACT OF THE DISCLOSURE A high vacuum gettering pump is described which is capable of providing a high vaporization rate of a getter material. The pump includes a high thermionic emission cathode filament disposed over a surface of a bulk supply of a getter material such as titanium. The getter material is contained within a cup-shaped heat shield and has a peripheral configuration minimizing the amount of material at the sides of the body of getter material which will not be melted by thermionic emission from the filament when a high operating potential is applied between the filament and the getter material body. A shield is also provided blocking line-of-sight travel of vaporized getter material to a portion of the filament which does not reach a high enough temperature to prevent condensation of the getter material. The shield is tapered inwardly and curved away from the filament adjacent an unshielded portion thereof to minimize the effect caused on the operation of the filament by the build-up of getter material on the shield.

BACKGROUND OF THE INVENTION This invention relates to high vacuum pumping and, more particularly, to a gettering pump capable of providing a high vaporization rate of a getter material and a consequent high pump speed.

It is a Well-known phenomenon that condensation of certain materials on a surface in a high vacuum system will result in a pumping action of residual gases in the system. Such pumping action is due to a chemical reaction of the gases with the condensing material and/or mechanical burying of the molecules of the residual gases by the condensing material. It will be appreciated that the pumping speed of such a getter material will be partially dependent upon the amount of such material which can be vaporized for condensation on a surface. Prior to the instant invention, it has not been economically feasible or practical to provide in a high vacuum system enough vaporized material to obtain with a getter pump, per se, the high speeds desirable to pump down large vacuum chambers. For this reason, the major use of the gettering phenomena in the high vacuum pumping art has been in getter-ion and sputter-ion pumps in which ionization of the gases to be pumped is utilized to aid in the gettering action. Such pumps are relatively complicated and limited with respect to the area which can be used as a getter pumping surface, however, in view of the necessity of providing the ionization. Therefore the art has generally been forced to use quite large and expensive difiusion pump systems in order to obtain the high pumping speeds necessary to pump large chambers to high vacuums.

BRIEF SUMMARY OF THE INVENTION The present invention provides a gettering pump capable of vaporizing large amounts of a getter material for condensation on a pumping surface with consequent high pumping speeds without the aid of ionization. For certain gases, the pumping speed obtainable with a pump of the present invention is over 5x10 liters per second at torr pressure. In its basic aspects, the pump includes a bulk supply of a getter material, such as titanium, to be 3,666,377 Patented May 30, 1972 vaporized, and heating means spaced from an exposed surface of the getter material for heating such surface to a temperature suflicient to cause vaporization of the material at the surface. In a preferred embodiment of the instant invention, the heating means also melts the material at the surface and, most importantly, the getter material body periphery adjacent the surface is designed so that there is a minimum amount of unmelted material laterally adjacent the melted material. This results in low heat dissipation from solid portions of the material with a consequent greater utilization of the heat to vaporize the material. Also, the heating means is desirably in the form of a cathode filament spaced above the surface and capable of providing the heating by electron bombardment of the surface. To assure that the filament remains sutficiently hot to cause the thermionic emission necessary to vaporize the necessary large amount of getter material, a filament shield is provided blocking line-of-sight between those portions of the filament which are not sufiiciently hot to prevent condensation of the getter material thereon. As will be explained more fully hereinafter, the filament shield is designed to provide the necessary shielding and yet minimize the effect on the operation of the filament of any build up of getter material on such shield.

The instant invention has other features which are important and aid in its ability to provide high pumping speeds without the necessity of providing an ionization means and which will be apparent from the following more detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the accompanying drawings:

FIG. 1 is a partly broken away side elevation view of a large vacuum chamber having a plurality of the pumps of the invention mounted therein for pumping the chamber to an ultra-high vacuum;

FIG. 2 is a partly broken away side elevation view of the area circled by the line 2-2 in FIG. 1 and illustrating a preferred embodiment of the pump of the invention;

FIG. 3 is an enlarged plan view of the pump illustrated in FIG. 2 taken on a plane indicated by the lines 33 in FIG. 2;

FIG. 4 is an enlarged cross-sectional view taken on the lines 44 of FIG. 2 and depicting the terminal connection for the filament of the pump;

FIG. 5 is an enlarged cross-sectional view of a portion of the pump of FIG. 2 illustrating the cathode filament and bulk supply of getter material;

FIGS. 6 and 7 are partial side and plan views, respectively, of the filament for the pump illustrating the filament shield therefor; and

FIGS. 8 and 9 are, respectively, side and plan views of a prior art cathode filament shield.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 depicts a large vacuum chamber, generally referred to by the reference numeral 11, used to outgas and test large satellites, such as conununication satellites, prior to their being orbited in space. Chamber 11 has a volume of about 300 cubic feet which must be pumped to, and maintained at, an ultra-high vacuum of 10- torr. A satellite placed within the chamber is dirty in the vacuum sense, i.e., it has not been outgassed and includes many organic parts, etc., which evolve gases at a fairly rapid rate as the chamber is pumped. In order to handle the evolved gases while at the same time lowering or maintaining the chamber pressure, the chamber pumping system must have a high pumping rate. For example, if the chamber 11 were to be pumped with a conventional high vacuum system employing diffusion pumps, at least four 48" diameter diffusion pumps with their attendant systems and high space requirements would be necessary. In contrast, two pumps of the present invention are capable of providing the high pumping speed and low pressures for all gases which may be present in the chamber except for the noble gases. To this end a plurality of pumps of the invention, generally referred to by the reference numeral 12, are mounted Within chamber 11 above a chevron baflle 13. As illustrated, pumps 12 are positioned below the inner surface of the dome 14, which surface acts as a condensing surface for the getter material vaporized by the pumps 12 and thereby provides a large getter pumping surface for the chamber. While a plurality 12 of gettering pumps are illustrated, it will be appreciated that only two pumps are activated at any one timetl1e others are included within the chamber to provide within the chamber pumps having a fresh supply of getter material without the necessity of opening the chamber.

A preferred embodiment of a pump of the instant invention capable of providing the high pumping speed and ultra-high vacuum necessary for the chamber 11 is illustrated in FIGS. 2-7. With reference first to FIG. 2, the pump 15 includes a bulk supply of a getter material 16, such as titanium, within a cup-shaped heat shield 17. Shield 17 may be of any suitable material as for example tantalum having good high temperature and heat shielding properties. A high thermionic emission cathode filament 18 is supported above the upper exposed surface 19 of getter material 16 at a location at which it will bombard the surface 19 with electron energy upon a suitable potential being applied between the filament and such surface.

Cup-shaped shield 17 is supported by a metal bracket 21 in an electrically conductive manner on a disc 22. Such bracket may be secured to the bottom surface of the cup 17 by means of brazing or the like and has outwardly flared flanges 23 secured to the disc 22 by bolts 24. To provide means for applying an electric potential to the getter material, an electrically conductive high voltage input rod 26 depends downwardly from disc 22 and extends through a mounting flange 27. Such rod is insulated from the flange 27 in any suitable manner, and terminates at its lower end in a conventional electrical vacuum feedthrough fitting 28 of the demountable type. Rod 26 thereby provides means for applying a high electrical potential to heat shield 17 and, hence to the upper surface 19 of the bulk supply of getter material.

Besides acting as a high voltage input for the getter material, rod 26 also supports the forward end of disc 22 above flange 27. A pair of rod supports 29 (only one of which is visible) supports the rear end of disc 22 on flange 27. As illustrated, each of the rods 29 is physically connected to the flange 27 by means of a high voltage insulator 31. The provision of the insulators 31 and the insulated manner in which input rod 26 passes through flange 27 electrically isolates such flange from the high voltage to be applied to the bulk supply of getter material. It is desirable that insulators 31 and the insulated feedthrough for voltage rod 26 be spaced a substantial distance from the getter material, as illustrated, to prevent the heat generated at such getter material from reaching and affecting the insulating properites of the insulators.

Means are also provided for supplying electrical power to cathode filament 18. More particularly, and as best illustrated in FIG. 3, filament 18 has two spaced apart terminal ends 32 and 33 secured respectively in connector sockets 3'4 and 36. Sockets 34 and 36 are, in turn, supported by vertically depending electrical lead rods 37 and 38, respectively. As illustrated in FIGS. 2. and 4, lead rod 38 is eelctrically and mechanically secured to an input rod 39 by a transversely extending coupling block 41. Input rod 39 extends downwardly and passes through mounting flange 27. Such rod is insulated from flange 27 and has at its lower terminal end a conventional electrical vacuum feedthrough 42 of the demountable type similar to the one on the end of high voltage rod 26.

Lead rod 37 secured to socket 34 is connected by a coupling block 43 (FIG. 4) to a cylindrical ground shield 44. As illustrated, ground shield 44 surrounds the electrical input rods to the filament and getter material source and is connected to mounting flange 27. When the pump of the invention is operated under high power as will be discussed below, the shield 44 has been found desirable to prevent the filament current from bypassing the getter material and striking the voltage input and support rods. Such shield is grounded and acts as an electrical barrier between the filament and the rods by collecting any electrons which may strike it, thereby preventing any high electron current from damaging the rods.

When it is desired to mount the pump of the invention in a vacuum chamber with the electrical feedthrough terminal ends 28 and 42 exterior to the vessel, mounting flange 27 can be provided with means for securing the same to the chamber wall in a vacuum tight manner. For example, such flange can be provided with one of the mating surfaces designed to receive a sealing gasket in the manner disclosed and claimed in US. Pat. 2,308,758. It will be appreciated, however, that in some instances, such as in the arrangement shown in FIG. 1, it is not necessary that the mounting be provided by a flange 27. For example, the socket box 45 illustrated in FIG. 1 for plurality of the pumps can be designed to suitably support the pumps without the necessity of such a flange. It will further be appreciated that in such an instance the terminal couplings 28 and 42 can be conventional high vacuum couplings rather than feedthrough couplings since the electrical members to which they are secured will also be within the vacuum chamber.

As mentioned previously, prior getter pumps of the general nature of the present invention have not been capable of vaporizing suflicient getter material to provide the necessary pumping speeds for large chambers and the like. The instant invention, however, includes several improvements and features which result in the pump being able to dispense up to 8 grams per hour of a getter material such as titanium. Such a high output of getter material enables the use of a large area of a condensing surface as a gas pumping surface, making possible high pumping speeds. For example, the instant invention is capable of pumping nitrogen at a rate of 5 X 10 liters per second at 10- torr pressure.

For an appreciation of the features providing the invention with this capability, reference is first made to FIG. 5 which illustrates the getter vaporization portion of the pump during its operation. When a high potential is placed between the filament 18 and bulk 16 of getter material, electron bombardment of the surface 19 by electrons emitted from filament 18 by thermionic emission causes the upper surface 19 of the getter material to melt and form a puddle 46 of melted material. Upon further electron bombardment of the surface 19, the melted material in puddle 46 vaporizes and condenses on a pumping condensing surface which, as is conventional, is exposed to the gases to be pumped. It has been found that the peripheral configuration of the bulk of material has a pronounced influence on the amount of getter material which can be vaporized. More particularly, applicants have found that the peripheral shape of the body of material laterally adjacent the puddle should desirably be such that the amount of unmelted material at the sides of the melted portion is minimized. This results in most of the heat applied to the surface 19 being utilized to maintain the surface melted and vaporize the melted material. Heat loss is thereby inhibited since there is no solid material adjacent the puddle 46 to dissipate and radiate heat which Would otherwise be advantageously utilized in vaporizing the material. Desirably, the general peripheral body shape of the getter material adjacent surface 19 should be defined by the peripheral profile at the surface 19 of the electron current having sufficient energy to at least melt the getter material when the pump is operating at or near full power. In the particular arrangement shown, it has been found that the optimum peripheral body shape is one in which the periphery of the body laterally adjacent the surface 19 tapers conically outward in the direction away from the filament. With such an arrangement, the unmelted body portion of the getter material will act as a crucible for the melted puddle as shown in FIG. 5, and yet the amount of material laterally adjacent the puddle is minimized.

As the surface being bombarded drops below the lip 47 of the cup-shaped heat shield 17, the peripheral shape of the body becomes less critical. Most heat energy which might be radiated from body 16 from a location below the lip 47 will be reflected back to the body by the heat shield. Thus, there is no appreciable heat loss to affect the amount of heat available to cause vaporization. For example, when the body has been vaporized to the surface indicated by the dotted lines 48 and 49 a large amount of solid material is located laterally adjacent the puddle but there is no appreciable heat loss due to such material. For this reason, the body need not be tapered for any distance below lip 47. In fact, it is desirable that a reverse taper be provided on the body for material savings, since as the surface of the material being vaporized gets beyond a certain distance from the filament 18, the lateral dimensions of the puddle tend to contract.

As will be evident from FIG. 5, the periphery of the material 16 is not continuously shaped in the desired taper but rather has an outwardly stepped configuration. This is because the material 16 is provided by stacking a plurality of plates 50 of the getter material and then passing axially therethrough a dowel 51 to secure such plates together. It has been found that when the bulk of material is so formed from a plurality of plates, it has lower axial heat transfer characteristics than it does when it is made from one solid mass of material. Heat can travel laterally through the material but at the interfaces between adjacent plates 50 it sees a series of barriers to axial transfer. The plate arrangement therefore results in lower upward travel and dissipation of heat and, consequently, the paddle 46 of melted material has a larger diameter than it would have if the bulk of material was provided in one solid mass.

Since it is only necessary that the periphery of the body have the general taper discussed above rather than a continuous taper, such taper can be most easily obtained by providing the uppermost plates 50 with a less diameter than lower ones as illustrated. Within the heat shield 17 the plates again can be of a less diameter.

Another difficulty had in the past in attempts to vaporize a large bulk of getter material by electron bombardment from a filament is that the vaporized material condenses on those portions of the filament which are not at a sufficiently high temperature to prevent such condensation. More particularly, the getter material tends initially to condense and build up on filaments adjacent their terminal ends. Built-up getter material on the filament ends cools the portions of the filament adjacent the ends and therefore also causes build up at the adjacent portions. In this manner condensed getter material tends to creep along the filament. Consequently, the thermionic emission from the filament is continually reduced, thereby preventing not only the vaporization of large amounts of the bulk material but, in time, preventing any vaporization at all.

It has been the practice in the past in order to combat this build up to provide a filament shield blocking line-ofsight travel of vaporized getter material between the bulk of material and the portion of the filament which during operation of the filament does not reach a temperature sufficiently high to prevent condensation thereon of the getter material. The getter material will then condense and build up on the shield rather than the filament. However, it has been found that the manner in which the material builds up on the shield deleteriously aflects the power distribution from the filament. This can be understood by referring to FIGS. 8 and 9 which illustrate a prior art shield 52 in place to protect the end 53 of a filament 54. As is illustrated, as the condensed material 55 builds up on the shield, the material tends to diverge outwardly from the shield surface on which it deposits. The resulting divergent body of material then blocks thermionic emission from portions of the filament not desired to be shielded, thereby reducing the amount of bombardment on the getter material with a consequent reduction in the amount of the vaporized material. The amount of reduction, however, is greater than one would expect considering the filament emission area which is shielded by the body of getter material on the shield. It is thought that the build-up body deleteriously affects the emission current distribution over a much greater area of the filament than that actually shielded with the consequent greater reduction.

As another salient feature of the instant invention, the shield for each of the terminal ends of the filament is designed to minimize the effect of build up of getter mate rial on the shield. More particularly, such filament shield is shaped to present a minimum surface area facing the bulk of the getter material on which vaporized getter material can condense and build up during operation of the pump. To this end, and as is shown in FIG. 7, the shield 56 associated with each of the filament terminal ends 32 and 33 is tapered inwardly in the direction away from the terminal end. The result is that as the filament shield approaches the area of the filament from which high thermionic emission can be expected, the filament shield presents less shielding of the filament. Most importantly, because of this tapered arrangement, the getter material 57 condensed and built-up on the filament will not shield an extended area adjacent the thermionic emission portion. It has been found that such built-up material 57 on the shield then does not appreciably affect the emission from the filament.

For best results, it is also desirable that the shield 56 be curved away from the filament adjacent the unshielded high thermionic emission portion thereof as is illustrated in FIG. 6. The getter material on the forward end of the shield then will extend vertically downward as shown in FIG. 6, rather than overhanging outwardly and forwardly as shown in FIG. 8. A further diminishment of the effect of the built-up material 57 on the high thermionic emission is thereby obtained. It is believed that the greater spacing of the forward end of the shield from the filament when the shield is curved also plays a part in the minimization of the effect of the built-up getter material on the thermionic emission.

It should be noted that each of the shields 56 is secured to the connector socket associated with the filament end it is to protect and includes a downwardly depending blocking flange 58 shielding from the getter material both the socket to which it is secured and its associated lead rod. The shield also includes upwardly extending sides 59 to prevent vaporized material from flowing around the lower shield surface and reaching the filament.

The cathode filament 18 of the invention is also designed to aid in the vaporization of a high amount of the getter material. That is, such filament is a wire filament having an extended length facing the getter material surface 19 so as to insure a high thermionic emission to such surface. As is illustrated in FIG. 3, the filament 18 is sinuously and reentrantly curved in a plane facing the surface to provide the extended length. Thus, thermionic emission portions of the filament 18 are closely adjacent one another in the lateral direction. This results in a high density of thermionic emission striking the getter surface to cause the increased vaporization.

As mentioned previously, up to 8 grams per hour of vaporized getter material can be provided with the instant invention for condensation on a pumping surface. Such a high rate of vaporization is obtainable by applying a 3 kv. positive D.C. accelerating voltage to high voltage input rod 26 and, hence, to the bulk of material, and applying an AC. potential in the order of -13 volts to the filament 18 via input rod 39. The pumping speed produced with such a high vaporization rate will depend, of course, upon the nature and conductance of the gases to be pumped and the area of the pumping surface on which the getter material will condense. For example, use of two pumps of the invention in the chamber 11 shown in FIG. 1 at the above potential settings will result in approximately 29,000 square inches of pumping surface on the dome of the chamber. With an appropriate chevron design for baffie 13 separating the pump from the working area of the chamber, the resulting obtainable pumping speed in the chamber will be 130,000 liters/ sec. at torr pressure for all getterable gases in the chamher.

It will be appreciated that in many instances and at certain pressures it is not necessary or desirable to operate the pump of the invention at a high power input. The amount of material vaporized will vary directly with the accelerating voltage applied between the filament 18 and the bulk 16 of material. Thus, if less material is to be vaporized, the voltage on the bulk of material can be lowered appropriately. The result is that less of the surface area of the bulk material is melted and less is correspondingly vaporized. If the pump of the invention is run at a low power for an extended period of time, a central cavity will be formed in the bulk of material having a diameter substantially less than that of the diameter of the material at the surface. It is therefore desirable to periodically increase the power applied to the pump whenever it is operated at a low power to melt the material at the surface 19 surrounding the cavity so that the newly melted material will flow into the cavity. This will result in a more uniform usage of the bulk supply of material.

While the pump of the invention has been described with respect to a preferred embodiment, it will be appreciated that variations are possible within the scope of the invention. For example, the pump can be combined with another pump, such as an ion pump, to obtain enhanced pumping.

What is claimed is:

1. A high vacuum gettering pump comprising a bulk supply of a gettering material to be vaporized, means spaced from an exposed, generally planar surface of said supply of gettering material for melting a portion of said material and heating said surface to a temperature sufficient to cause vaporization of said material at said surface, the periphery of said body laterally adjacent said surface tapering generally outwardly away from said surface, and means for mounting said pump within a vacuum system with said exposed surface of said getter material in the line-of-sight of a getter material condensing surface exposed to the gases to be pumped.

2. The high vacuum gettering pump of claim 1 wherein said means for melting said portion of said getter material comprises a cathode filament spaced from said exposed surface, and means are provided for connecting electrical power between said filament and said surface of said getter material to cause said heating of said surface by flow of current between said filament and said surface.

3. The high vacuum gettering pump of claim 1 wherein said bulk supply of getter material is contained within a cup-shaped heat shield and said means for heating said surface of said supply is spaced above the open end of said cup.

4. The high vacuum gettering pump of claim 2 wherein said bulk supply of getter material is contained within a cup-shaped heat shield which is supported by an elongated high voltage power input rod, said filament being spaced above the open end of said cup in facing relationship to said gettering material surface, and a ground shield is provided surrounding said high voltage power input rod to shield the same from the flow of current from said filament.

5. A high vacuum gettering pump comprising a bulk supply of a gettering material to be vaporized, a cathode filament spaced from an exposed surface of said supply of gettering material, means for connecting electrical power between said filament and said surface to cause heating of said surface to a temperature suflicient to cause vaporization of said material by flow of current between said filament and said surface, and a filament shield blocking line-of-sight travel of vaporized getter material between said surface and a portion of said filament which during operation of said pump does not reach a temperature sufficient to prevent condensation of said getter material thereon, said filament shield being shaped to present a minimum surface area facing said surface of said getter material on which vaporized getter material can condense and build up during operation of said pump, and means for mounting said pump within a vacuum system with said surface of said getter material in the line-of-sight of a getter material condensing surface exposed to the gases to be pumped.

6. The high vacuum gettering pump of claim 5 wherein said shield is curved away from said filament adjacent an unshielded high thermionic emission portion thereof.

7. The high vacuum gettering pump of claim 5 wherein said filament shield blocks line-of-sight between said getter material surface and a portion of said filament adjacent a terminal end thereof, said filament extending from said terminal end along said filament toward a high thermionic emission portion thereof and being tapered inwardly in the direction away from said terminal end to present a minimum area adjacent said high thermionic emission portion blocking line-of-sight between said filament and said getter material surface.

8. A high vacuum gettering pump according to claim 7 wherein said shield is curved away from said filament from adjacent the terminal end thereof.

9. The high vacuum gettering pump of claim 5 wherein said filament is a wire filament having an extended length facing said getter material surface to provide high thermionic emission to said surface.

10. The high vacuum gettering pump of claim 9 wherein said wire filament is sinuously curved in a plane facing said surface of said getter material and has two terminal ends, each one of which has a filament shield as defined blocking line-of-sight between said gettering material surface and the portion of said filament adjacent its associated terminal end.

11. The high vacuum gettering pump of claim 5 wherein said bulk of getter material has a peripheral body shape adjacent said surface minimizing the amount of unmelted material laterally adjacent a melted portion thereof.

12. The high vacuum gettering pump of claim 11 wherein said filament shield blocks line-of-sight between said getter material surface and a portion of said filament adjacent a terminal end thereof, said filament extending from said terminal end along said filament toward a high thermionic emission portion thereof and being tapered inwardly in the direction away from said terminal end to present a minimum area adjacent said high thermionic emission portion blocking line-of-sight between said filament and said gettering material surface.

13. The high vacuum gettering pump of claim 12 wherein said shield is curved away from said filament adjacent an unshielded high thermionic emission portion thereof.

14. A method of operating a high vacuum gettering pump having a large bulk of getter material to be heated by electron bombardment comprising the steps of bombarding a first portion of an exposed surface of said bulk of material with sufficient electron energy to raise said surface portion to a temperature causing vaporization of said material thereat, and periodically melting the material at a second portion of said surface adjacent said first portion and causing flow of said melted material from said second portion over said first portion to replace the material vaporized at said first portion, whereby utilization of a majority of said gettering material is assured.

15. The method of claim 14 wherein said periodic melting of said material at said second surface portion 10 is caused by bombarding said second portion with electron energy.

References Cited UNITED STATES PATENTS 5/1966 Connor 23069 4/1968 Lloyd 23069 X ROBERT M. WALKER, Primary Examiner 

